Common sense is learned

I don't think I believe in "common sense" as it is usually invoked, and before I start, this is not a rant about how "people are stupid these days". This is more about what we mean when we say "common sense" and why we get so surprised when someone doesn't behave the way we expect.

As I understand it, "common sense" refers to a set of learned observations about how the world works, and our ability to apply that knowledge to new situations. This can go wrong in a few ways. First, someone's experiences may differ significantly from yours, making their observations of effects very different from yours, making them interpret events unusually. For instance, imagine that someone has only ever seen other drivers speed up at yellow lights, to beat the red, and has never had any other guidance on the meaning of that colour of traffic signal. What would their "common sense" interpretation be? Well, obviously, yellow means "speed up". It's just common sense: everyone does that. They are wrong in their conclusion, but they are being 100% consistent with the evidence they have observed, and therefore they are being sensible.

When we are surprised by an interpretation like that, what is actually happening? We bring a lot of observations to the table when we try to make sense of an event, and those are different for all of us. Our common sense when we are very young and still learning how a lot of the world works becomes very different as we grow up and form a more complete world view. But since that world view is formed through experience and education, it is different for everyone, making everyone's common sense a little bit unique.

Mokalus of Borg

PS - This is my excuse for why I'm a little weird.
PPS - Feel free to use it for yourself, too.